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Attic Ventilation Formula

1 square foot (or 144 square inches) of net free area (NFA) ventilation for every 150 square feet of attic space.

The single most important variable in determining the longevity of a roof system is maintaining a properly ventilated attic. Having a properly ventilated attic requires 50% intake and 50% exhaust of air flow. Use the 1 in 150 Rule as shown below (hover over the tooltip to see calculation):

500 square feet of attic space

240 square inches

600 square feet of attic space

288 square inches

700 square feet of attic space

336 square inches

800 square feet of attic space

384 square inches

900 square feet of attic space

432 square inches

1000 square feet of attic space

480 square inches

1100 square feet of attic space

528 square inches

1200 square feet of attic space

576 square inches

1300 square feet of attic space

624 square inches

1400 square feet of attic space

672 square inches

1500 square feet of attic space

720 square inches

1600 square feet of attic space

768 square inches

1700 square feet of attic space

816 square inches

1800 square feet of attic space

864 square inches

1900 square feet of attic space

912 square inches

2000 square feet of attic space

960 square inches

Example of how to calculate net free area:
1200 square foot attic divided by 150 square feet = 8 square feet (1200 / 150 = 8)
8 square feet times 144 square inches = 1152 square inches (8 x 144 = 1152)
1152 square inches divided by 2 = 576 square inches at soffit and at exhaust (1152 / 2 = 576)

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Attic Ventilation Formula

1 square foot (or 144 square inches) of net free area (NFA) ventilation for every 150 square feet of attic space.

The single most important variable in determining the longevity of a roof system is maintaining a properly ventilated attic. Having a properly ventilated attic requires 50% intake and 50% exhaust of air flow. Use the 1 in 150 Rule as shown below (hover over the tooltip to see calculation):

Attic Ventilation Using The 1 in 150 Rule

Soffit Net Free Area

Exhaust Net Free Area